1. Field of the Invention
The present invention relates to a fuel injection system for an internal combustion engine, and more particularly to a fuel injection system for an internal combustion engine having an exhaust gas recirculation system.
2. Description of the Prior Art
A sequential fuel injection technique is known in which a fuel injection valve is located near each intake valve in the associated intake passage and is controlled to inject an amount of fuel required for one power stroke, according to engine load and engine rpm. The fuel injection is made into the intake passage after the intake valve has been opened and during the intake stroke, thereby correctly adjusting the air-fuel ratio of the mixture in the associated combustion chamber, and improving fuel economy and exhaust emissions. It has been discovered, however, that vaporization and atomization of the liquid injected fuel in such a fuel injection system are inferior to what is experienced when using a carburetor. For this reason, it has been proposed to arrange the fuel injection timing at the end of an intake stroke in each cylinder so that liquid fuel stays in an intake port a relatively long period, thereby promoting vaporization and atomization of the liquid fuel, see, for example, Japanese Unexamined Patent Publication No. 57 (1982) -108428. However,arranging the fuel injection timing all the time at the end of an intake stroke is disadvantageous because the aforementioned merits of sequential fuel injection are then destroyed. Also, an exhaust gas recirculation technique in which a part of exhaust gases is recirculated to an intake passage to reduce harmful NO.sub.x (nitrogen oxides) is known. In the exhaust gas recirculation system, recirculated exhaust gases can lower combustion temperature because exhaust gases are inert, lowering combustion temperature leads to suppressing generation of NO.sub.x. Therefore, exhaust emissions can be improved under all engine load conditions by appropriate use of exhaust gas recirculation technique. However, exhaust gas recirculation has the disadvantage of lowering combustibility of the mixture. Accordingly, an engine having a sequential fuel injection system and an exhaust gas recirculation system must resolve the dichotomy between maintaining adequate combustibility and preserving the merits of sequential fuel injection.